X-ray tube

ABSTRACT

An X-ray tube wherein the glow cathode coils are located outside of the direction of the vertical projection of the focal point upon the cathode body. The invention is particularly characterized in that the striking location of the vertical projection of the focal point upon the cathode body is made for the absorption of ions.

United States Patent 1 Friedel X-RAY TUBE [75] Inventor: Rudolf Friedel,Erlangen, Gemiany [73] Assignee: Siemens Aktiengesellschaft, Munich,

Gennany 22 Filed: Feb. 19, 1974 21 App|.No.:443,864

[30] Foreign Application Priority Data Feb. 28, l973 Germany 23l006l[52] US. Cl 313/60; 313/55 [51] Int. Cl. H01] 35/06 [58] Field of Search313/60, 55, 57

[56] References Cited UNITED STATES PATENTS 2,691,869 3/l954 Atlee3l3/60 [451 May 20, 1975 Primary Examiner-Rudolph V. Rolinec AssistantExaminerDarwin R. Hostetter Attorney, Agent, or Firm-Richards & Geier[57] ABSTRACT An X-ray tube wherein the glow cathode coils are locatedoutside of the direction of the vertical projection of the focal pointupon the cathode body. The invention is particularly characterized inthat the striking location of the vertical projection of the focal pointupon the cathode body is made for the absorption of ions.

5 Claims, 7 Drawing Figures X-RAY TUBE This invention pertains to anX-ray tube wherein the glow cathode coils are located outside of thevertical projection of the focal point upon the cathode body.

The load capacity of a rotary anode in short time space is determined bythe greatest permissible focal point temperature, which is limited byreaching the melting point of the anode material. However, in actualexperience the maximum focal point temperature must be considerablylower to provide a stable tube operation and sufficient life duration.In addition to a few problems soluble by constructive means, such as,for example, the steaming of glass container with the resultinginsufficient high tension firmness, this has substantially the followingtwo reasons:

1. Independently from the melting point steam pressure between anode andcathode must not be so high that gas discharging takes place. A measurefor this is the free passage length of electrons.

2. Anodes can be dirtied" during manufacture, further treatment and alsoduring operation in the tube, for example, by coatings on the outersurface which have a higher steam pressure than the anode material.

Although the free passage lengths of electrons at usual maximumtemperatures of the focal point of about 2,500 C or greater for tungstenanodes are substantially larger than the electrode spacing, tubeinstabilities can arise. Examinations have shown that responsible forthat is a rear action upon the cathode. In the electronic current of thecathode atoms are ionized, for example, metal steam out of the focalpoint or nonremovable gases. The positive ions are then accelerated uponthe heating coil of the cathode where they cause a rise in current byheating of the coil surface, secondary electron formation and reductionof space charge in front of the coil. This leads to increased steamformation, ionization, current increase, etc.

The damage thereby produced appears typically as periodic meltings ofthe focal point path which are produced in the stated manner incombination with the generator (inner resistance, capacities). Thisproblem becomes substantially more important for tubes with high outputsince the inner resistance of the generator must be held small due tothe necessity select as low as possible the empty run voltage fortechnical and expense reasons, and even eliminate the inner resistancein wide operational ranges.

In a known device, high load X-ray tubes have arrangements wherebyelectrons have the form of a hollow ray. Furthermore, a special strikingsurface for ions is provided in the center of the cathode for absorbingthe ionic strike penetrating into the ray, instead of allowing it tostrike the part emitting the electrons. However, this is a complicatedand expensive solution, since it is necessary to produce a hollowelectronic ray which moreover cannot be used in medical X-ray technologydue to representation quality. Furthermore, the cathode must also have aspecial construction with a striking section for ions enclosed by theelectron source or the tubular electronic path. In addition, in thisconstruction the cathode body is located only partially in the verticalprojection of the ring-shaped focal point, it also goes past it. Partsof the cathode emitting electrons are also struck by the projection, sothat the abovementioned damaging effects can still take place, at leastpartly.

On the other hand it is known in X-ray tubes to guide the electronic raybundle emerging from the glow cathode screw in a curved path upon aspecific point of the anode and there possibly to hold it, for example,even during a rotary movement of the entire tube. Then the sscrew islocated outside of the vertical projection of the focal point. However,no means are provided which would cause the ions to strike the cathodebody and their absorption. The ions run without being controlled andstrike, for example, the wall of the container or parts of electrodes,which is a drawback, since this may produce metal coatings whichdiminish the high voltage firmness of the tube.

An object of the present invention is to provide constructive meanswhich would eliminate the abovementioned detrimental cathode backeffect, which would provide greater operational safety of X-ray tubesused in medical field and which would increase their load capacity.

In the accomplishment of the objectives of the present invention it wasfound desirable to make the striking location of the vertical projectionof the focal point upon the cathode so that it will absorb ions.

Due to this arrangement the ions strike a location of the cathode bodywhich is located to the side of the glow coil and on which they canproduce no damage. Thus is avoided an influencing of the sensitive coil,namely, an incontrolled increase of electronic emission (FIG. 2).

In a simple embodiment of the present invention the striking location ofthe ions is a plate which melts with difficulty and is well heatconducting, such as a plate of tungsten, having a thickness of 0.5 to 5mm (FIGS. 3, 4, 5 and 7). The thickness of the plate is of lessimportance since it is only important for stability and heat capacity.It can consist of tungsten, molybdenum or tantalum. An improvement canbe provided if the plate is placed into the cathode body (FIG. 4). Therecess should have at least approximately the shape of the focal point.Due to this arrangement despite the high melting point and small gaspressure of tungsten, etc., the tungsten gas produced by the strikingions can not get into the range of the electronic ray or to locationswhich would affect the high voltage firmness of the tube.

The cathode head can be also held entirely out of the projection line ofthe focal point (FIG. 6). However, then it is necessary to use acomplicated cathode system, for example, an electronic gun which hasmagnetic and electric lenses causing a sharply bundled electronic flatray to drop upon the anode at an inclination, and to provide a separateion receiving system.

Electrons produced by a heat coil or some other electronic emitter aredeviated by an electric field to bent half-circular paths so that theoutgoing direction and the striking direction of the electronic ray uponthe anode differ from each other by more than 0 and a maximum of Thisprovides that the heating coil is even more effectively protected fromionic striking.

The same effect is produced by deviating the ions by means of apermanent or electric magnet (FIG. 5). If the striking plate consists oftantalum, zirconium, titanium, etc., when the plate is heating duringionic striking in addition a getter effect is produced which isadvantageous for the operation of the tube. The space in which the plateis located is enclosed by metallic walls, so that various effects, forexample, the coating of walls with metal, are limited to this space andcause no dam age.

The invention will appear more clearly from the following detaileddescription when taken in connection with the accompanying drawingshowing by way of example only, preferred embodiments of the inventiveidea.

IN THE DRAWINGS FIG. 1 is a side view of an X-ray tube.

FIG. 2 is a diagrammatic section extending transversely to FIG. 1through the cathode and anode of the tube shown therein.

FIG. 3 is a diagrammatic section showing a different cathode arrangementwherein a plate of heavy metal is provided upon the striking location ofthe ions.

FIG. 4 is a diagrammatic section showing a cathode arrangement wherein aplate of heavy metal is lowered into the body of the cathode.

FIG. 5 is a diagrammatic section illustrating the deviation of an ionicray by means of a magnetic field.

FIG. 6 is a diagrammatic section showing the shooting of electrons by anelectronic gun in an inclined direction upon the anode.

FIG. 7 is a diagrammatic section through a cathode arrangement whereinthe electronic ray is deviated upon a curved path by 90.

FIG. 1 shows a vacuum casing l of the tube which contains the cathodedevice 2 and the anode device 3. The cathode device 2 located at one endof the casing includes the outer casing 4, the mounting, namely, theactual cathode body 5, and the glow coils 6 and 7 which are actuated bylines 8, 9 and 10. Electrons leaving the coils 6 and 7 strike the rotaryanode 11 which is connected by the axle 12 with the rotor 13.

According to the present invention a side shifting of the coils and sucha shaping of the focus milling is provided, that there is a deviation ofthe electronic ray 14 upon the focal point 15 of the anode 11, as shownin FIG. 2. The path 16 of ions shown by broken lines strikes the cathodebody 5 at the location 17 outside of the coil 6.

According to FIG. 3 upon this location of the cathode can be placed aplate 18 consisting of tungsten which opposes a good resisting force tothe striking ions.

In the construction shown in FIG. 4 the plate 19 consisting ofmolybdenum is arranged in a depression 20 so there is substantial spaceseparation from the heating coil and the range of the electronic ray.

The arrangement shown in FIG. 5 corresponds to a substantial extent tothat of FIG. 4. In it at the rear end of the opening 20 there is amagnetic field 21 which is used to guide the ionic ray 22 upon the plate23 consisting of tantalum which is arranged in the striking location.The plate 23, as is also the case in the construction of FIG. 4, islocated in a cover 23, shown by broken lines in FIG. 5 constituting ahollow space enclosed by metal walls, in which will be deposited thematerial given up by the plate 23.

The construction of FIG. 6 differs from the earlier described devices inthat it does not require a special deviating device for the electronicray 24, since this ray emerges with great acceleration from theelectrode gun 25 and strikes the anode 26 in an inclined direction, sothat particles emerging from the anode or ions released in space willnot strike the cathode system. The striking location of ions indicatedby 25 is constructed in the same manner as in embodiments shown in otherfigures and is connected to or separately from the gun 25 in a mannerwhich is not illustrated.

In the construction shown in FIG. 7 the shape of the cathode body 27produces an electric field which devi ates the electronic ray 28 fromthe glow coil 29 by about to the anode 30. Ions leaving he focal pointof the anode 29 pass through the hole 31 and stirke the plate 32 made oftungsten which is located in a separate hollow space 33 in a mannersimilar to that shown in FIGS. 4 and S. The side walls of the space 33constitute a tubular support 34 for the plate 32.

I claim:

1. An X-ray tube comprising an anode body. a cathode body and at leastone incandescent cathode coil located on said cathode body to project acathode ray beam forming a focal spot on said anode body and located outof the area onto which said focal spot is projected in rectangulardirection from its surface and which area is shaped to absorb ions, saidarea being fu rnished with a heat-conducting body of refractorymaterial.

2. An X-ray tube according to claim 1, wherein said material is selectedfrom the group consisting of tungsten, molybdenum and tantalum.

3. An X-ray tube according to claim 1, wherein said area is located in ahole provided in the cathode body.

4. An X-ray tube according to claim 3, wherein the bottom of said holeis provided with a heat Conducting refractory body.

5. An X-ray tube according to claim 3, wherein the hole penetrates thecathode body totally and said area is located on a heat conducting bodyof refractory material suppoted on the rear of the cathode body.

1. An X-ray tube comprising an anode body, a cathode body and at leastone incandescent cathode coil located on said cathode body to project acathode ray beam forming a focal spot on said anode body and located outof the area onto which said focal spot is projected in rectangulardirection from its surface and which area is shaped to absorb ions, saidareA being furnished with a heat-conducting body of refractory material.2. An X-ray tube according to claim 1, wherein said material is selectedfrom the group consisting of tungsten, molybdenum and tantalum.
 3. AnX-ray tube according to claim 1, wherein said area is located in a holeprovided in the cathode body.
 4. An X-ray tube according to claim 3,wherein the bottom of said hole is provided with a heat conductingrefractory body.
 5. An X-ray tube according to claim 3, wherein the holepenetrates the cathode body totally and said area is located on a heatconducting body of refractory material suppoted on the rear of thecathode body.